499 light-seconds

Just above the Sun's surface is a layer called the chromosphere. Looking like a field of fiery grass, this layer can be seen only by using a special filter that lets through only one wavelength of light, a reddish colour given off by ionised hydrogen gas (hydrogen alpha). This artificially-coloured image also shows an optical effect known as limb brightening, where the edge of the Sun appears brighter than the middle.

Credit: Image made in red light by Robert Gendler.